1. Field of the Invention
The invention provides a process for producing polycrystalline silicon.
2. Description of the Related Art
Polycrystalline silicon (polysilicon for short) serves as a starting material in the production of monocrystalline silicon by means of crucible pulling (Czochralski or “CZ” process) or by means of zone melting (float zone or “FZ” process). This monocrystalline silicon is divided into wafers and, after a multitude of mechanical, chemical and chemo-mechanical processing operations, used in the semiconductor industry for manufacture of electronic components (chips).
More particularly, however, polycrystalline silicon is required to an increased degree for production of mono- or multicrystalline silicon by means of pulling or casting processes, this mono- or multicrystalline silicon serving for manufacture of solar cells for photovoltaics.
The polycrystalline silicon is typically produced by means of the Siemens process. In this process, in a bell jar-shaped reactor (“Siemens reactor”), thin filament rods (“thin rods”) of silicon are heated to surface temperatures of 900-1200° C. by direct passage of current and a reaction gas comprising a silicon-containing component, especially a halosilane, and hydrogen is introduced via injection nozzles. In the course of this, the halosilanes break down at the surface of the thin rods. This deposits elemental silicon on the thin rods from the gas phase.
The silicon rods were held in the reactor by special electrodes which generally consist of high-purity electrographite. Every two thin rods connected to the voltage with different polarity at the electrode holders are connected by a bridge at the other end of the thin rods to form a closed circuit. Through the electrodes and the electrode holders thereof, electrical energy is supplied to heat the thin rods.
During the deposition, the diameter of the thin rods grows. At the same time, the electrode, beginning at its tip, grows into the rod base of the silicon rods.
The main material used for the electrodes is graphite, since graphite is available in very high purity and is chemically inert under deposition conditions. Moreover, graphite has a very low specific electrical resistivity.
After the attainment of a desired target diameter of the silicon rods, the deposition process is ended, and the glowing silicon rods are cooled down and deinstalled.
Subsequently, the U-shaped rod pairs of polysilicon obtained are typically cut to length at the electrode and bridge ends and comminuted to chunks. The comminution is effected by means of a crusher, for example with a jaw crusher. Such a crusher is described, for example, in EP 338 682 A2. This is optionally preceded by a pre-comminution by means of a hammer.
Previously, the graphite electrode is typically removed. EP 2 479 142 A1 discloses removing at least 70 mm from the electrode end of the rod. This is said to lead to a lower concentration of extraneous substances such as chromium, Iron, nickel, copper and cobalt in the interior of the silicon chunks produced. The removal is effected by means of a cutting tool, for example by means of a rotary saw. However, a not inconsiderable amount of polycrystalline silicon is lost in this process.
However, there are also known processes in which the removed end of the rod, comprising silicon and graphite, is treated chemically by etching the graphite away or converting into a powder form which can be removed easily from the polysilicon. This gives rise to a rod piece which has been freed of graphite and can be processed further. However, there is the risk of contaminating the polycrystalline silicon in the process. Processes of this kind are described in CN 101691222 B, CN 101974784 A, CN 102121106 A and CN 102211773 A.
In CN202358922 U, an attempt is made to prevent growth of the electrode into the rod base through a suitable construction of electrode and electrode holder. This is said to lead to a higher yield of polycrystalline silicon.
The problem addressed is thus that of completely removing the electrode while achieving minimum contamination of the polycrystalline silicon. In addition, the process is to ensure high productivity and a maximum yield of polycrystalline silicon.